1. Field of the Invention
The present invention relates to a method and apparatus for controlling the air-fuel ratio in an internal combustion engine.
2. Description of the Related Art
One prior art apparatus for controlling the air-fuel ratio in an internal combustion engine includes means for calculating a base fuel amount signal during a steady state of the engine in correspondence with values of predetermined engine operation parameters, including engine coolant temperature; means for detecting a transient operation state of the engine representing output power increase demand; means, responsive to the detected engine temperature and the detected transient state of the engine, for generating a reinforce promotion signal which has an initial value determined by the detected transient state of the engine and which is increased by a factor changing toward unity at a rate decided by the detected engine temperature; and means for supplying fuel to the engine in accordance with the base fuel amount signal and the reinforce promotion signal to supply the engine with fuel. This type of apparatus enables a fuel supply system with a constantly optimum air-fuel ratio not only in a steady state but also in a transient state of the engine and thus enables a constantly optimal engine operation. Such an apparatus is disclosed, for example, in Japanese Unexamined Patent Publication (Kokai) No. 56-6034.
In the above-mentioned type of apparatus, however, no consideration is given to long-term changes in the operating characteristics of the engine, for example, changes in characteristics due to deposition of a viscous material such as fine carbon particles originating from lubricant constituents and combustion products at the valve clearance or at an injection nozzle of an electronic fuel injector and changes in characteristics due to such deposition at the rear surface of each cylinder intake valve. In addition, the above-mentioned apparatus has no means for detecting a change of the air-fuel ratio during a transient state such as an acceleration mode or a deceleration mode deviated from the optimum value due to the long-term changes in the operating characteristics of the engine, changes in the gasoline characteristics, or the like. Therefore, if gasoline having low volatility characteristics is used, or if long term changes occur in the engine, the air-fuel ratio becomes lean during an acceleration mode, thereby leading to bad drivability such as non-smooth acceleration. Contrary to this, if gasoline having high volatility characteristics is used, the air-fuel ratio becomes rich during a deceleration mode, thereby increasing the fuel consumption and deteriorating the emission gas characteristics.
Clogging of injectors may be compensated for by a feedback operation by an air-fuel ratio sensor in the case of a steady state but this has not been possible in a transient state due to the absence of correction means. Also, this type of apparatus does not take into consideraton inevitable variations in and aging of the structures of the manufactured engines or airflow meters.
Further, it does not consider the problem of the seasonal difference in specific properties of the gasoline used. Usually, a gasoline producer sells different kinds of gasoline for each season of the year. These, of course, differ in volatility characteristics, as expressed by Reid vapor pressure or distillation characteristics. Even gasolines from the same producer vary from 0.5 kg/cm.sup.2 to 0.86 kg/cm.sup.2 in vapor pressure or from 40.degree. C. to 58.degree. C. in 10% recovered temperature. Such differences in volatility characteristics result in considerably different air-fuel characteristics in the transient operation state, and no consideration is given to fluctuations in the air-fuel ratio due to these changes of volatility characteristics of gasoline.
Thus, when engine operation characteristics change due to long-term deposits or when low volatility gasoline is used, the air-fuel ratio in an acceleration state becomes relatively lean. Hence, the engine operation deteriorates, e.g., non-smooth acceleration occurs. On the other hand, the air-fuel ratio in a deceleration state becomes relatively rich. Hence, emission and the specific fuel consumption deteriorate. Even when a high volatility gasoline is used, the air-fuel ratio becomes rich in an acceleration state, resulting in the same problems.
A technique for the control of the air-fuel ratio to overcome the above problems has been proposed in Japanese Patent Application No. 58-129497 (corresponding to U.S. Ser. No. 630,682), however, this still requires further improvement. According to this technique, the air-fuel ratio deviation from a reference air-fuel ratio is detected during the transient period of the internal combustion engine, and the correction amount for transient fuel injection amount correction is calculated in accordance with the detected air-fuel ratio deviation, thereby avoiding the deviation of the air-fuel ratio from the optimum value due to the deposition of viscous material on the rear surface of each cylinder intake valve, the clogging of the injectors, the aging of the engines, the airflow meters, and the like, and thus, the drivability, the fuel consumption, and the gas emission are improved.